The present invention relates to filtering membranes on the basis of welded polymer structures and to a method for manufacture such membranes. The membranes of the present invention are intended for separation of liquid and gaseous mixtures and may find application in chemical industry, bioengineering, medicine, food industry, etc.
At the present time, separation of liquid and gaseous mixtures with the use of separation membranes find wide application in various industries. Thus, such membranes are used in the chemical industry for separation of azetropic mixtures, cleaning and concentration of solutions, cleaning and isolation of high-molecular compounds from solutions that contain low-molecular components, etc. In bioengineering the separation membranes are used for isolation of active substances, such as vaccines and ferments. In food industry such membranes find application in the production of juice concentrates, milk, and high-quality sugar. In oil industry the separation membranes are used for separation of gaseous products from wastes for synthesis of polymers. Other fields for efficient use of separation membranes are treatment of water and solutions, purification of potable water and sewages, etc.
One of greatest achievements in the development of membrane technique was a membrane method for desalination of sea water invented by S. Loeb and S. Sourirajan in 1960 (S. Loeb and S. Sourirajan, UCLA Dept. of Eng. Rep. 60-60 [1960]). This method (hereinafter referred to as L-S method) determined for many years the strategy in development of membrane production technology. The authors of this method, S. Loeb and S. Sourirajan, were awarded a Nobel Prize. Many years of experience gained in the improvement of the original L-S method are reflected in many patents and publications. The double-layered structure proposed by S. Loeb and S. Sourirajan was composed of a thin dense cellulose acetate film applied onto a thin porous substrate. The upper dense layer with a thickness not exceeding 1 xcexcm possessed selectivity to water and therefore allowed isolation of sodium chloride therefrom. The porous substrate with a thickness exceeding 100 xcexcm imparted mechanical strength to the structure. If both layers are made from the same polymer, the membranes are called anisotropic membranes (AM), and if they are made of different materials the membranes are called composite membranes (CM). Depending on their shapes, the membranes can be flat, spiral, tubular, or in the form of hollow fibers (First Demonstration of Reverse Osmosis by UCLA SEAS. See in Internet: http://www.engineer. Ucla. Edu/history/osmosis. Html).
Modern membranes comprise thin hollow fibers with the walls made from a porous material without any selectivity. Selectivity is acquired by coating the membrane walls with another material, which possesses selectivity and determines productivity of the membrane operation.
Processes of membrane separation of fluids, which find industrial application, are the following: microfiltration, ultrafiltation, and reverse osmosis. These processes are realized with the use of semipenetrable two-layered membranes produced in the form of AM and CM structures with different sizes of pores.
Microfiltration retains particles in the range of 0.1 to 10 microns (1,000 to 100,000 Angstroms). Particles in this range, such as paint pigments or bacteria, are retained and concentrated by the membrane. Microfiltration can be used to remove bacteria and small suspended solids or clarify beverages.
Ultrafiltration retains particles in the range of 0.001 to 0.1 microns (10 to 1000 Angstroms). Protein and sugar molecules are in this size range. Ultrafiltration can be used to reduce the biochemical oxygen demand (BOD) of waste water by removing substances such as sugar. Ultrafiltration can also be used to separate oil from waste water so that the oil may be recycled.
Reverse osmosis makes it possible to retain particles as small as 0.001 microns (10 Angstroms) or smaller. It also retains ionic substances such as dissolved salts or metal ions, Reverse osmosis can be used to concentrate rinse water from plating operations. The concentrated rinse water then can be used to replenish the plating bath.
It is known that porosity of membrane produced in accordance with the L-S method has the following values expressed in percentages: microfiltrationxe2x80x9470%; ultrafiltrationxe2x80x9460%; reverse osmosisxe2x80x9450% (Ralph E. White, Peter N. Pintarno. Industrial Membrane Processes: AIChE Symposium Series. American Institute of Chemical Engineers, 1986, 82, No. 248, pp. 98-108, 255-262).
As can be seen from the above, that only a part of the membrane volume is active, and the smaller the dimensions of pores, the smaller is the active volume of the membrane. This is because the L-S method has limitations with regard to distribution of pore density, and the pores themselves have arbitrary distribution. More specifically, the structure of polymer membranes consists of crystalline and amorphous zones, and structural stability of such membranes depends on links between molecular chains inside the crystalline areas, whereas filtration of a substance occurs mainly through more porous amorphous zones. In microporous membranes filtration occurs through the interlinked pores inside the membrane. Thus, if efficiency of a membrane is determined in terms of its porosity, the arbitrary nature of porosity distribution can be considered as an additional contribution to decrease in membrane""s efficiency because of twisting and elongation of the diffusion path. In actual membranes, the diffusion path is much longer than the membrane thickness. The L-S method in principle does not allow to increase the amorphous part in the volume of the active zone of the membrane or to make it comparable with the membrane thickness and thus to improve permeability. This is an essential disadvantage of the L-S method.
Another disadvantage of double-layered L-S membranes consists in that the porous substrate shields the active zone of the membrane and reduces its working area. Even though in many cases porosity of the permeable substrate is much higher than that of the active zone of the membrane, as one of the separable components is removed from the system, a relatively thick substrate presents a significant resistance to the transfer process.
Let us consider some particularities inherent in the manufacture of membrane filters by he L-S method.
AM and CM for Reverse Osmosisxe2x80x94Reverse osmosis can be carried out with both AM (on the basis of cellulose ethers, polyamides, and polysulfones) and CM (on the basis various polymers on substrates from polysulfone or polyethyleneterephalate). An AM membrane of this type consists of a substrate having a thickness of about 100 microns and an active layer having a thickness of about 0.2 microns. The membrane is produced by a dry, spontaneous, or a wet coagulation method (see E. Drioli, M. Nakagani. Membranes and Membrane Processes. N-Y.; Plenum Press, 1986, p. 115-187). In the dry method, a polymer, e.g., a cellulose ether or an ether mixture, is dissolved in a solvent, such as acetone. The solution is combined with pore-forming agents, water, and glycerol.
The solution is poured onto a substrate, and the solvent is gradually evaporated. In the wet method, the solution, which contains polymer such as a cellulose acetate, a pore-forming agent (magnesium perchrorate), water, and an organic solvent (acetone, methylethylketone, and methyl or ethyl alcohol), is applied in the form of a thin layer onto a glass or metal plate. Prior to application, the solution and the plate are cooled in a cooling chamber to a temperature from xe2x88x928xc2x0 C. to xe2x88x9216xc2x0 C. After application of the solution, the plate is dried for several minutes and is immersed in cold water (0xc2x0 C.). Following 1 hour retention in cold water, the solution is washed out from the coating and the coating is gelatinized. The film with an anizotropic structure, i.e., a thin surface layer on a microporous substrate, is removed. The longer the solvent evaporation time, the thicker is this active layer.
Disadvantages of the membrane manufacturing process described above are a multiple-state processing, difficulties of control, the use of organic solvents, and difficulties of cleaning the membranes.
At the present time, CMs consisting of a 75-micron thick porous substrate and a thin (300 to 1000 Angstrom) active layer find wide practical application. Substrates used in such membranes are made mainly of polyethyleneterephthalate and ppolysulfone. The membranes are produced by polycondensation of the monomer mixture on a porous substrate (see R.E. Kesting. Synthetic Polymer Membranes. N.Y.; John Wiley and Sons, 1985, p. 29-41).
KMs of another version consist of a sulfonated polyfurane on a substrate from a sheet of microporous polysulfone. The substrate is produced by passing a polysulfone solution through an orifice into a coagulation bath. The sulfonated polyfurane is applied in the second stage of the process in the following manner. The substrate is coated with a composition that contains a furfural alcohol, sulfuric acid, a polyethyloxide isopropanol, and water. Slow heating to 150xc2x0 C. causes cross-linking and formation of a non-soluble sulfonated polyfurane. A disadvantage of such membranes consists in complicated and difficult manufacture.
All known membranes described and criticized above are structurally the same and differ from each other only by materials, methods of manufacture, and porous structures. For illustration of specific structures of known membranes we can refer to several patents listed below.
U.S. Pat. No. 6,177,011 issued in 2001 to H. Hachisuka discloses a composite reverse osmosis membrane having a separation layer with polyvinyl alcohol coating. The membrane has a high salt rejection, a high water permeability, and a high fouling tolerance, and permits practical desalination at a relatively low pressure. The membrane is provided by coating the surface of a reverse osmosis membrane of aromatic polyamide with polyvinyl alcohol (PVA), for example, and controlling the surface zeta potential of the separation layer within.+0.10 mV at pH 6. This reverse osmosis composite membrane is electrically neutral and controls the electrical adsorption of membrane-fouling substances having a charge group present in water. Next, the embodiments of the present invention will be described below with reference to the drawings. The membrane consists of a supporting porous layer, a separation layer applied onto the porous layer, and a thin protective layer of polyvinyl alcohol as the surface layer of the separation layer.
U.S. Pat. No. 5,500,247 issued in 1996 to P. Hagqvist describes a web-like starting material formed by combining mutually two webs of membrane-layer carrier sheets and an intermediate web of the spacing layer or sheet. The starting material may also include only one web, which comprises two outer carrier layers that are integrated with an inner spacing layer. In the latter cases, the starting material web will suitably include a thermoplastic fiber fabric for the spacing layer, wherein the outer carrier layers are formed by heat-treating the surface of the fiber fabric in a manner to obtain a densified carrier structure adapted for the application of the membrane layer.
Japanese Laid-Open Patent Application No. 2001-129329 issued in 2001 to K. Nagatsuka describes a reinforcing material for a polytetrafluoroethylene (PTFE) membrane filter which flame resistant and consists of a porous reinforced web and a thin PTFE film.
Although the aforementioned structures differ from each other by construction of various elements, all of them have an active member of the membrane blocked by a support or protection member, to which the membrane film is attached by gluing, fusing, or welding.
It is an object of the present invention to provide membranous filters which have controlled distribution and orientation of pores, short filtration paths, efficient operation, simple construction, and low cost. Another object is to provide filtering membranes, which can be manufactured in a single-stage process. Another object is to provide a simple single-stage method for manufacturing the aforementioned membranes.
The filtering membranes of the present invention are made from a pair of polymer films stretched in a liquid medium for the formation of crazes filled with the aforementioned medium. The crazed films are perforated and then stack together in a stretched or released state and are welded together into a sealed structure with a plurality of parallel welding seams arranged, e.g., in mutually perpendicular directions, so that the seams form a plurality of cells. Each aforementioned perforation is arranged in each cell so that perforations of one film alternate with perforations of another film. In other words, the membrane formed by the aforementioned method consists of a plurality of sealed cells having on one side of the membrane perforations opened to the input side of the membrane and with perforations on the other side of the membrane open to output side of the membrane. Adjacent cells are interconnected only through the welding seams.
Welding can be carried out by contact heating or with the use of a laser beam, or the like. The substance captured inside the crazes may comprise a dispersion medium used for fixing dimensions of the crazes or a substance for treating the fluid being filtered. In operation, the medium to be filtered diffuses from the input cells to the output cells of the membrane through the material of the welding seams. Filtering is enhanced if the material of the seams has aforementioned crazes. In such a membrane, the total length of the welding seams on the area of 1 m2 may reach several hundred thousand meters. The membrane possesses selectivity sufficient for separation of two or multiple-component mixtures in a single-stage process. Depending on the method used for preparation of the polymer-film surface prior to welding, the membrane filter may have efficiency from 5 to 40,000 kg/m2xc3x97D.